1. Technical Field
The present invention relates to processes for production of isocyanates by the metal-catalyzed reaction of formamides and oxygen.
2. Description of the Prior Art
Heyboer, U.S. Pat. No. 4,207,251, discloses a method for gas phase production of C.sub.1-24 organo-isocyanates involving oxidative dehydrogenation of corresponding N-monosubstituted formamides. This reaction is extremely exothermic; therefore, some method for controlling reaction temperature must be employed to maintain reaction temperatures below those which would accelerate side reactions, or lead to decomposition of product isocyanates. For example, Heyboer discloses use of small diameter reactors, or introduction of large volumes of an inert gas to reaction mixtures to absorb liberated heat.
Rao et al., U.S. Pat. No. 4,537,726, disclose use of at least two adiabatic reaction stages which are linked in series. The quantities of oxygen fed to each reactor are limited in order to control process temperature.
Disadvantages attend each of the foregoing heat-control strategies. Use of small reactor diameters or multiple reactors increases equipment and engineering costs, and complicates process operation. When dealing with lower-boiling isocyanates, adding large volumes of inert gas (e.g., more than 50 volumes of inert gas per volume of oxygen) greatly increases the cost and expense of separating the desired product from the reactor effluent stream.
The improvement of the present invention permits partial or complete displacement of inert gas added to reactant streams for cooling, and use of single-stage reactor systems of conventional design. Accordingly, both capital and operating costs associated with isocyanate production can be greatly reduced. The improved process of the present invention is particularly suited for production of methyl isocyanate.